Gas anesthesia has been used during medical and dental surgery for many years. Over a century ago, the discovery and use of ethyl ether as an anesthetic began. In fact, gas anesthesia has been used extensively because of the ready availability of ethyl ether. Later it was discovered that nitrous oxide (N.sub.2 O) had anesthetic properties and mixtures of oxygen and nitrous oxide are widely used today as anesthetics. Gas anesthesia provides certain advantages over drugs generally administered internally or intravenously in that it can be administered in continuous applications of minute quantities.
While all anesthetic substances, including the gaseous types, have some effect upon a patient, excessive amounts can be dangerous as they may adversely affect the functioning of vital organs. Therefore, it is important to be able to control precisely not only the amount of anesthetic administered to a patient but also the rate of gas flow and the percentage of a particular gas with respect to the total gas flow where more than one gas is employed.
As the administration of an anesthetic to a patient is so critical anesthetics are often administered along with oxygen, the oxygen being supportive to the functioning of a patient's vital organs. By changing the percentage of anesthetic and the percentage of oxygen it is possible to control the degree of anesthesia. Moreover, the anesthetic gas may be shut off and pure oxygen may be fed to the patient.
Gas anesthesia devices have been under development throughout this century. These devices, for the most part, have been directed toward the delivery of two or more gases through some type of mixing apparatus to a breathing mask positioned over the mouth and nose of a patient. Very early designs utilized pressure bottle supplies of oxygen and nitrous oxide or some other gaseous anesthetic. These two gases were often made available to the patient through a crude mixing valve structure via piping from each supply cylinder. A simple gate valve was also used at the supply cylinder for each gas.
Improvements to these designs concerned themselves with the metering of pressures of the gases in the administering apparatus and with the mixing of the two gases in certain proportions. As the previously available mixing valves were quite crude, another improvement in anesthesia apparatus was brought about by the introduction of the needle valve. Such needle valves provided a much more precise metering of the volume of gas passed through the valve as a function of the valve position. These needle valves permitted a calibration of the cross sectional area of the valve opening as a function of the number of revolutions the needle valve is rotated. Further improvements have incorporated check valves which permit the passage of gases in one direction only. These check valves prohibit the backing up of one gas into the other side of the system when the gas pressures are unequal.
The development of the single control, dual (or ganged) needle valve ushered in a more modern era of gas anesthesia apparatus. This valve comprises two needle valves, one for regulating oxygen flow and one for regulating anesthetic flow. These gauged, dual needle valves are mechanically linked together through a shaft and drive which rotate the needle valve shafts in opposite directions. The valves may be rotated to provide a proportional mix between oxygen and the anesthetic gas, wherein, at one extreme adjustment, the oxygen flow is fully open and the anesthetic gas flow cut off, and at the other extreme, the anesthetic gas flow is fully open and the oxygen flow is cut off. A control knob may be provided which is calibrated as a function of the percentage of each gas being administered to the patient. More precisely, the rotation of the control knob may be noted against a scale and correlated to read percentage of anesthetic being administered.
Koehn, U.S. Pat. No. 3,128,764 discloses an apparatus which supplies a mixture of nitrous oxide and oxygen to a patient. Koehn, as well as others, have used flow meters positioned in respective gas lines to provide a visual indication of rate of flow of the gas flowing therethrough.
The flow meters used by Koehn, as well as the flow meters incorporated into the apparatus discussed below, are of conventional design.
Flow meters usually comprise a vertically extending transparent tube having a ball therein of a specific gravity greater than the specific gravity of the gas being measured. The annular space between each ball and the inner surface of its respective tube and the velocity of the gas flowing upwardly through the tube determines the drag forces acting on the ball. When the stream of flowing gas produces a drag force on the ball equal and opposite to the force of gravity acting on the ball, the ball with reach a position of static equilibrium in the tube. As the flow rate varies, the position of the ball in the tube varies accordingly so that the particular position of the ball in the tube indicates a particular flow rate. A calibrated scale is disposed adjacent the tube and indicates the rate of flow of the respective gas as measured by the ball within the tube.
While these improvements have advanced the state of the anesthesia art, the development has not yet reached the point where these devices are sufficiently presice and simple and reliable. Even with these improvements in anesthesia apparatus, mistakes continue. These mistakes, while sometimes due to failures in the equipment, are often due to mistakes on the part of the operator to properly adjust or monitor the operation of the equipment, perhaps while acting under adverse conditions, or perhaps while under stress or a state of exhaustion and most certainly while his attention is at least partially diverted to additional activities which must be performed simultaneously with the operation of the anesthesia equipment. Analog equipment of the type discussed above, has not proven entirely satisfactory.
The need persists for reliable, easy to operate, and easy to adjust anesthesia apparatus. Presently the operator must deal with an analog reading of his adjustment knob. His interpretation of the setting is a function of how finely he can read the analog scale, as well as, from what angle he is viewing the scale. It is well known that the interpretation of an analog scale will vary with the angle from which it is read.
An object of this invention is to provide a flow control system for an anesthesia apparatus having a direct, mechanically operated, on-line gas flow rate adjustment and a visual display of the instantaneous adjustment, wherein this display provides a digital read out of flow rate.
A second objective of this invention is to provide such apparatus wherein said display is driven by a calculation component responsive to the flow rate adjustment.
Another objective of this invention is to provide such an apparatus having at least two such on-line gas flow rate adjustments for individually and independently adjusting the flow rate of each gas wherein a display of percentage gas mixture is presented in an alphanumeric digital display.
Another object of this invention is to provide such an apparatus having separate visual displays for each flow rate adjustment, including selectively a digital readout of flow rate, wherein each display is individually driven by a separate calculation component responsive to a separate flow rate adjustment.
A further object of this invention is to include calculation circuitry responsive to all of the separate flow rate adjustments for driving a separate visual display as representative of the percentage of a single gas flow to the total gas flow.
An even further object of this invention is to provide a digital encoder as part of each separate flow rate calculation component and a percentage calculation circuit as part of the percentage gas flow circuitry.
A further object of this invention is to provide a pressure sensor interlock which permits flow of the anesthesia gas only in the presence of the flow of oxygen.
Another object of this invention is to provide pressure limit alarm sensors on both supply inputs to the flow control apparatus, these sensors alarming when the supply gas pressures are greater than or less predetermined pressure limits.